This invention relates to a method of treating a subterranean formation of non-uniform permeability, and more particularly concerns a method of reducing the permeability of subterranean formations to water thereby promoting better control of fluid injection patterns in the secondary or tertiary recovery of hydrocarbons and achieving water reduction in producing wells and thus reducing the quantity of water recovered from a subterranean formation penetrated by a well bore whereby the relative production rate of the hydrocarbons is increased.
Since only a portion of the oil contained in a subterranean reservoir can be recovered by primary methods, it has become general practice to employ various secondary or tertiary recovery techniques to produce the additional quantities of oil not economically recovered by primary methods. Of the various secondary or tertiary recovery methods available, one of the most widely practiced techniques is the displacement of oil from the reservoir with a driving fluid such as a floodwater injected for that purpose. Normally, in carrying out the flooding process, input or injection wells are utilized. These wells can be old existing wells or can be wells which are newly drilled into the oil-producing strata. The location of the injection wells with reference to the production wells is selected to afford a desired flood pattern, the selected pattern depending in part upon field conditions, the locations of existing wells, and the operator's preference. Aqueous drive fluids, such as water, brine, or a viscous aqueous fluid are forced into the input wells under pressure, and out into the surrounding oil bearing strata towards the producing well or wells. While waterflooding has been rather widely practiced in recent years, it is not without considerable operating problems and economical limitations particularly those associated with low oil recoveries in proportion to the amount of water injected.
Various surfactant and solvent floods have been proposed as means for recovering additional quantities of oil over that recoverable by conventional waterflooding. These processes, however, face serious operating problems when practiced in heterogeneous formations containing strata or channels having permeability substantially higher than the bulk of the formation.
One of the major problems encountered in a flooding operation is the breakthrough of the flooding medium from the flood front to the producing well relatively early in the displacement process and rapidly increasing producing water/oil ratios following the initial breakthrough. These difficulties result from the displacing medium channeling or fingering through the oil-bearing structure to the producing well, thus bypassing large zones of the oil-bearing strata. The reason for the channeling of the flooding medium to the producing wells and the resulting low oil recovery is due in part to the peculiar structure of the oil-bearing strata. Underground oil reservoirs, in most cases, consist of layers of sand or rock and, since no reservoir rock is perfectly uniform in composition and structure, the permeability will vary across the rock face or strata. Also, fractures, cracks, and other abnormalities can promote channeling of the displacement of the fluid.
In the normal flooding operation, maximum oil recovery is obtained when the driven fluid flows in a wide bank in front of the driving fluid which moves uniformly towards the producing well. To keep this bank of oil intact and constantly moving towards the producing well, a substantially uniform permeability must exist throughout the strata. If this uniform permeability does not exist, or is not provided, the flooding fluid will seek the areas of high permeability, and channeling occurs with the subsequent loss of some driving fluid energy and the appearance of excessive amounts of driving fluid in the producing well. Moreover, as the more permeable strata are depleted, the driving fluid has a tendency to follow the same channels and further increase the consumption of the flooding medium to the point where the process becomes uneconomical. It is, therefore, desirable to operate at a drive fluid to oil ratio that is as low as possible.
Another problem associated with the production of oil from oil-bearing formations containing highly permeable water channels or communicating with a water zone is the intrusion of water into the production well. Not only does this water intrusion cause production and disposal problems, but more importantly the beneficial effect of the natural water drive is at least, in part, lost thereby adversely affecting oil recovery.
It is advantageous to reduce the permeability of the water channels so as to render the formation more uniformly permeable and to increase the unit efficiency of the water drive, or alternatively to shut off the water intrusion.
Many processes have been proposed for reducing the permeability of a subterranean formation. For example, U.S. Pat. No. 3,308,885 discloses introducing an aqueous solution of water-soluble polyacrylamide into a subterranean formation around a well bore to reduce the water/oil ratio of the formation effluent.
A difficulty with the injection of a polymer solution into a subterranean formation is the high pressure required to inject the polymer at the well bore. The high pressure required to inject a viscous polymer solution at the well bore increases the risk of damage to the formation by fracturing.
U.S. Pat. No. 3,490,533, which is assigned to assignee of the present invention and is hereby incorporated by reference, discloses a method of introducing a polymerizable solution into a subterranean formation around a well bore comprising the injection of a polymerizable monomer dissolved in water and containing a polymerization catalyst having a latent period into the well bore, pressurizing the water containing monomer and polymerization catalyst so that it flows from the well bore into the surrounding formation and permitting the monomer to polymerize within the formation after the latent period of the catalyst has expired.
Although the method of U.S. Pat. No. 3,490,533 has been successful in introducing polymer solutions into the formation without damage to the formation, certain problems remain. The bottom hole temperature of a formation varies with geographical location of the formation and the depth of the formation. A number of formations have temperatures greater than 160.degree. F. and the polymer solution of U.S. Pat. No. 3,490,533 is not particularly stable at temperatures greater than 160.degree. F.
It is therefore, desirable to provide a method of treating a subterranean formation of non-uniform permeability to reduce the permeability of the formation to water which is effective at temperatures greater than 160.degree. F. and which can be introduced into the formation without damage to the formation.
The method of the present invention resides in a method for reducing the permeability of a subterranean formation having a temperature greater than 160.degree. F. and penetrated by at least one well bore by contacting the surfaces of the formation with an aqueous polymerizable composition. The method of the invention can be used in either the injection of waterfloods, polymer floods, or CO.sub.2 floods or hydrocarbon production wells for the purpose of reducing the water/oil ratio produced therefrom; e.g., reduce the mobility of water in the well bore area.
According to the present invention, the permeability to water of a subterranean formation is reduced by contacting the formation with an aqueous polymerizable composition comprising a polymerizable monomer, a polymerization catalyst having a latent period, and an alcoholic compound selected from the group consisting of a cyclic ether containing a hydroxymethylene substituent on a carbon atom bonded to the ether oxygen, an aliphatic alcohol having a carbon-to-carbon double bond or a carbon-to-carbon triple bond, and mixtures thereof. The aqueous polymerizable composition containing the polymerizable monomer, the polymerization catalyst having the latent period, and the alcoholic compound are allowed to flow from the well bore into the surrounding formation to contact the surface of the formation and the monomer is allowed to polymerize within the formation after the latent period of the catalyst has expired to form as a final product the desired viscous but still flowable aqueous composition. The alcoholic compound increases the heat stability of the polymer produced in the aqueous composition. The heat stability of the polymer produced in the aqueous composition has been found to be greater than the heat stability of a polymer produced by polymerizing the polymerizable monomer using the polymerization catalyst and subsequently adding the alcoholic compound to the polymer.
The use of the method of the present invention results in a reduction in permeability of the subterranean formation. Furthermore, the resulting polymer composition is effective at high temperatures including temperatures greater than 160.degree. F. and even as high as 250.degree. F. or more.